The exemplary embodiment relates to the digital imaging arts. It finds particular application in conjunction with a method and apparatus for incorporating watermarks comprising small marks in blank portions of pages of documents. The watermarks may be used for embedding information in hard copy documents which may be used to distinguish authentic hardcopy documents from counterfeit documents or for other applications.
Since print systems have been in existence, printers have sought methods for inhibiting counterfeiting and unauthorized copying of printed documents. Machine readable information in the form of watermarks, barcodes, and the like has been embedded into images on paper for a variety of applications, including document identification and authenticity verification. The code is generally invisible or visually unobtrusive and may be decoded by a device which is capable of reading the information. Current counterfeit prevention systems are frequently based on the use of digital watermarks. Digital watermarking is a technique which allows a user to add information (e.g., copyright notices, security codes, identification data, etc.) to digital image signals and documents. Such data can be in a group of bits describing information pertaining to the signal or to the author of the signal (e.g., name, place, etc.). Most common watermarking methods for images work in spatial or frequency domains. However, such techniques, while suitable for digital documents, are often not sufficiently robust to allow detection of the marks in hardcopies, i.e., when the digital document is rendered in physical form.
Additionally, many existing techniques are designed for a particular type of images, for example continuous-tone pictures or text. It is often difficult to select one algorithm for embedding a watermark on a page to be printed if the image type of the page is unknown. Another challenge for the existing technology is simplicity. Many methods require very complex calculations, either in embedding or in detection, and quite often, in both. Yet another challenge is to provide an embedding method which can be applied to both color images and black and white images. Some existing methods rely on embedding information in the yellow separation. They cannot be used for black and white printing devices.
There remains a need for alternative systems and methods to provide watermarking techniques which may be used for identification of images and/or documents, for uses such as prevention of counterfeiting, and for other purposes.